With the pending completion of the Human Genome Project, scientists will soon know the sequence of all human genes. This knowledge will create an unprecedented need for new rodent models to identify and understand gene function to test new therapeutic strategies aimed at alleviating human diseases. Maintenance of the exponentially increasing numbers of these mutant strains as live animals will not be feasible. Cryopreservation and genome banking of spermatozoa and oocytes (gametes) and embryos will be mandatory. However, there are two major impediments to genome banking of rodent models. Many mutant mouse and rat lines are infected with microbial pathogens. An understanding of the risks of transmission of infectious agents through gametes and embryos must be developed. Preservation of gametes and embryos will be compromised if they are contaminated with agents that lead to infection in the regenerated mutant lines and cause outbreaks of infectious disease. Methods to efficiently freeze mouse embryos and regenerate strains stocks from frozen stocks via embryo transfer are well established for the mouse, but little work has been done in developing efficient rat embryo cryopreservation methodology. The Specific Aims of this application are to: 1) determine which of the commonly found murine pathogens contaminate mouse and rat gametes and embryos; 2) develop methods to remove microbial pathogens from rodent gametes and embryos and thus, decrease the likelihood of their transmission during genome banking; and 3) develop improved rat embryo cryopreservation methods.